System for performing spectral analyses under computer control

ABSTRACT

Measurements of physical attributes such as dielectric film thickness that are susceptible to spectral analysis are accomplished rapidly and accurately by a spectrophotometric system in which a programmed digital computer operating concurrently with the optical scanning means automatically performs the calibrating, normalizing and data reducing functions that otherwise must be carried out as time-consuming human, mechanical or analog electronic operations. The control over the optical data handling operations exercised by the computer eliminates the need for mechanically or electronically adjusting the optical apparatus to meet changing system conditions, whether periodic or aperiodic. Source light is transmitted through a rotating variable-wavelength interference filter which acts during one-half of its cycle to transmit light of varying wave-length through a fiber-optic reference path directly to the optical data acquisition apparatus, while acting in the next half-cycle to transmit light of such varying wavelength indirectly to said data acquisition apparatus through a measurement path. In the present example, where film thickness is the attribute being measured, the measurement path comprises a bifurcated fiber-optic bundle, one branch of which is used to carry the light of variable wavelength to the sample, and the other branch of which carries light reflected from the sample to the aforesaid data acquisition apparatus. A computer program enables light passed through the reference path in one half-cycle to calibrate the system for measuring optical transmission or reflectance in the next half-cycle. Reduction of relative reflectance data to absolute reflectance data (needed for the accurate determination of film thickness) is accomplished by additional computer programs whose algorithms are based upon the discovery that all graphs of absolute reflectance versus wavelength for film samples of a given material having different thicknesses are bounded by a common pair of wave envelopes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to optical measuring techniques whichemploy spectrophotometers and the like. Specifically, it relates to themeasurement of dielectric film thickness by observing the variation ofinterference effects produced by varying the wavelength of the incidentlight.

2. Description of the Prior Art

A technique commonly used for measuring the thickness of a thindielectric film (e.g., silicon dioxide, silicon nitride, or photoresistsused on silicon in the fabrication of integrated circuits) is to observethe manner in which the intensity of light reflected from the film andits substrate varies due to optical interference effects as thewavelength of the incident light is varied. For given film and substratematerials, the distance between adjacent maximum or minimum points onthe reflectance curve is theoretically a function of film thickness. Inpractice, however, there are likely to be deviations from thetheoretical relationship due to the variation of the indices ofrefraction of the film and substrate with optical wavelength. Inaddition, unless the spectrophotometer is fully corrected for spectralvariation of its light source, aging of its electronic components,mechanical wear, and calibration of its optical paths, furtherdeviations from theory occur. In the conventional measurement, thelocations of the maximum and minimum points are read by a human from thecurve plotted by the spectrophotometer. These data points are then usedto calculate the thickness of the dielectric film. While this methodonly requires relative reflectivity (rather than absolute reflectivity)in order to make the measurement, it requires intervention of a human tofind the maximum and minimum points (with the associated unreliabilityand inaccuracy of human observers) and also utilizes only a small amountof the measured data while demanding equal accuracy at all points. Inthe conventional spectrophotometric system used for this purpose, theoptical apparatus has numerous electrically and mechanically adjustableparts which must be adjusted both periodically and aperiodically tocompensate for system variations of the kind mentioned above. All ofthese adjusting, compensating and calculating functions customarily areaccomplished by time-consuming human and mechanical operations, whichgreatly impede the measurement of film thicknesses.

SUMMARY OF THE INVENTION

An object of the present invention is to relieve both the opticalapparatus and the human operator of the need to perform mechanicaladjustments and complex off-line calculations, thereby simplifying theconstruction of the optical apparatus, reducing the job requirements ofthe operator and greatly speeding up the measurement process. Thisobjective is accomplished by a novel system design which enables aprogrammed digital computer to make instantaneous interpretive orcompensating adjustments of the acquired optical data as needed in lieuof the cumbersome adjusting techniques conventionally employed for thispurpose as described above. Among the features of the present systemwhich enable it to operate in this efficient computer-controlled modeare the following:

Visible light from a light source is passed through a rotatingvariable-wavelength interference filter which is effective in one halfof its rotation to pass a variable-wavelength monochromatic beam througha reference path to a photomultiplier in the data acquisition circuitryand is effective in the next half-cycle to direct such a beam through asample measurement path. This sample measurement path comprises abifurcated fiber-optic bundle, one branch of which directs thevariable-wavelength light onto the film sample to be measured, and theother branch of which passes light reflected from the sample to theaforesaid photomultiplier. The computer, which processes digitizedversions of the optical data acquired by the photo-multiplier throughthe two paths just described, makes corrective modifications of the datareceived through the measurement path in accordance with data receivedthrough the reference path. No adjustments need be made to the opticalsystem per se, which therefore may have a relatively simpleconstruction.

For the film thickness measurement the data acquired will generally berelative reflectance data, relieving the stringent requirement on sampleplacement needed to obtain absolute reflectance data. The computerimmediately converts this to the corresponding absolute reflectance databy the execution of novel programs whose algorithms are derived from theprinciple (newly propounded herein) that all curves of absolutereflectance versus wavelength are bounded by the same upper and lowerlimiting curves or wave envelopes for any given film material regardlessof film thickness, and that any relative reflectance curve can beconverted to its corresponding absolute reflectance curve by a simpleproportionating function which relates the tangency points on therelative reflectance curve to the aforesaid wave envelopes. Once thisreduction has been accomplished, the determination of film thickness isa straightforward computation.

In other embodiments (not shown herein) the system can operate with thesample measurement path arranged not for direct reflectance as justdescribed but for non-normal incidence reflectance, scatteredreflectance, or transmission mode. The data acquisition and correctionin such an instance is carried out as described above with the exceptionthat the algorithm for converting from relative to absolute reflectancedescribed above is valid only for thin films of known optical propertieson known substrates. The system can be used in a fully automatic mannerin a variety of applications. The key feature is the use of aspectrophotometer in combination with a digital computer which can storeand compare information, make calculations based upon known theory, makecomparisons to standards, plot results, print output results, make adecision based upon them, and cause processes to be modified. Examplesof such applications are the measurement of color characteristics offoodstuffs, measurement of thickness of photoconductor layers making useof their optical absorption and reflectance characteristics, the controlof color printing processes, measurement of temper of heat treated steelsheets, etc. The computer controlled instrument lends itself to rapidmeasurement and data interpretation in applications where real timeresponse is needed to control a process or to verify that control isbeing maintained.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention, as illustratedin the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general schematic representation of a computer-controlledspectrophotometric system for measuring dielectric film thickness inaccordance with the invention.

FIG. 2 is a more detailed showing of the fiber-optic sample measurementpath included in the apparatus of FIG. 1.

FIG. 3 is a view similar to FIG. 2 illustrating the ability of themeasurement path to function effectively despite changes in the angularorientation of the sample.

FIG. 4 is a diagrammatic showing of the manner in which data of variouskinds is entered and processed in the computer to obtain the desired endresult, i.e., the thickness of the film being measured.

FIGS. 5A to 5E, when assembled in the manner indicated by FIG. 5,constitute a flowchart of the computer program which is utilized forcarrying out the invention in the present embodiment.

FIG. 6 is a set of graphs showing the relationship between thewavelength of the monochromatic incident light and the absolutereflectance of the sample for films of different thickness made of thesame material.

FIGS. 7A and 7B, when assembled vertically in the order named,constitute a printout of the DATAI subroutine included in the computerprogram.

FIGS. 8A through 8F, when assembled vertically in the order named,constitute a printout of the THIKI subroutine included in the computerprogram.

FIGS. 9A through 9D, when assembled vertically in the order named,constitute a printout of the ORDR subroutine included in the computerprogram.

FIGS. 10A and 10B, when assembled vertically in the order named,constitute a printout of the THINI subroutine included in the computerprogram.

DESCRIPTION OF PREFERRED EMBODIMENT

In FIG. 1 there is illustrated in a general schematic way aspectrophotometric system which is designed to measure the thickness ofa thin dielectric film sample F in accordance with the principle of theinvention. The system consists of three main parts -- an opticalapparatus 10, a data acquisition apparatus 12 and a programmed digitalcomputer 14. The optical apparatus 10 is of simple construction, lackingthe usual complex adjusting mechanisms (strings, cams, levers, movingslits, etc., or their analog electrical equivalents) that arecharacteristic of conventional spectrophotometers. The data acquisitionapparatus 12 has the important function of transferring to the computer14 the optical data generated by the apparatus 10 in a chronologicallyordered digital form which enables such data to be readily assimilatedby the data storing and processing facilities of the computer 14. Thepower of the computer 14 then is utilized to perform at high speed manynecessary functions which otherwise would have to be accomplished inmuch greater time and with far less facility and accuracy by mechanicalmovements, analog electrical adjustments and/or human efforts in orderto obtain the thickness measurement or other end result desired.

The optical apparatus 10 furnishes a beam of varying-wavelengthmonochromatic light alternately through two available fiber-opticconducting paths to a photomultiplier 16 (or comparable light detectingdevice) in the data acquisition apparatus 12. One of these alternatelight paths is a reference path 18 which leads directly to the detector16. The other is a bifurcated measurement path 20, one branch 26 ofwhich conducts the probing light beam to the film sample F whosethickness or other characteristic is to be measured, the other branch 28of this path then conducting light which is reflected or transmitted bythe sample to the photomultiplier 16. The wavelength of themonochromatic light beam varies periodically through a specified rangeof wavelength values, the rate at which it varies with respect toelapsed time being substantially the same in each of these periods. Thearrangement is such that during one period the specified wavelengthspectrum is scanned while the light beam is passing through thereference path 18, and during the next period it is scanned while thelight beam passes through the measurement path 20, this cycle beingrepeated without interruption. The output signal of the photomultiplier16 therefore represents, in electrical analog form, the variation ofdetected light intensity with respect to wavelength (which in turn is afunction of elapsed time) during those nonadjacent periods while thelight is passing through the reference path 18, interspersed with thevariations of detected light intensity versus wavelength during theintervening periods while the light is passing through the measurementpath 20.

The analog output signal of the photomultiplier detector 16 is convertedby an analog-to-digital converter 22 to a corresponding train of codeddigital signals that are passed to an interface 24, which enters theminto the digital computer 14. The operation of the data acquisitionapparatus 12 is intermittently timed or synchronized by the opticalapparatus 10 (in a manner which will be explained subsequently herein)so that each digital value entered into the computer 14 has both anumerical significance denoting light intensity and a wavelengthsignificance, the relative positioning of these entries with respect toone another determining the wavelength value associated with eachnumerical entry. With the data being entered in this form, the computer14 can readily construct tables or "curves" of light intensity versuswavelength for both of the optical paths 18 and 20, from which it ispossible to perform all of the calibrating, normalizing and datareducing functions that are needed to determine the thickness of thefilm sample without imposing any of these burdens upon the opticalapparatus 10 or a human operator.

The data acquisition apparatus 12 is of essentially conventionalconstruction and will not be disclosed in detail herein. The fact thatit is able to perform the novel and important function described aboveis due to the manner in which it is operated by the optical apparatus 10for transmitting to the computer 14 the type of interspersed referencedata and measurement data that are needed by the computer forcalibrating and normalizing purposes.

The computer 14 is preferably, but not necessarily, a small ormedium-scale computer such as an IBM Type 1130 data processing system,for example, which is programmed to perform a variety of operations uponthe raw data fed into this computer by the data acquisition system 12.The program under which the computer 14 operates has a number of novelsubprograms, the most significant of which are considered herein undertwo functional headings: (1) calibration and normalization, and (2) datareduction. These subprograms enable the computer 14 to perform atelectronic speeds and without human intervention many functions thatotherwise must be accomplished by the use of much slowerinstrumentalities and techniques. The computer is able to "capture inflight," as it were, the time and wavelength-based reference andmeasurement data generated by the optical apparatus 10, storing thisdata in a manner such that the wavelengths associated with the variousoptical entries are denoted by the relative positions or addresses whichthese entries assume when they are stored in the computer memory. Fromthese stored tables of light intensities and wavelengths, the computerprogram (to be described hereinafter) will derive all of thecorrectional information and computed data needed to achieve the desiredend result, which in this case is to determine the thickness of thedielectric film sample F. The system as it presently is being practicedcan measure film thicknesses up to 50,000 angstroms with a precision ofwithin 10 A throughout most of this range.

Referring now to FIG. 2, which shows in magnified form the part of themeasurement path 20 which lies in the vicinity of the film sample F, thepath 20 in this particular embodiment of the invention comprises abifurcated fiber-optic bundle having an initial entrance branch 26 forconducting the incident light to the sample F and a subsequent exitbranch 28 for conducting the light reflected from the sample and itssubstrate (not shown) to the light detecting means (e.g.,photomultiplier 16, FIG. 1). The fibers in the junction 30 of the twobranches or legs 26 and 28 have a "randomized" distribution, the fibersbelonging to branch 26 being interspersed as evenly as possible withthose belonging to the branch 28. There is substantially no couplingbetween the input fibers in bundle 26 and the output fibers in bundle 28in the absence of a sample or other reflecting surface at the junction30.

An advantage of using fiber optics in the light guiding path 20 is thatonly the light impinging the sample F at or near normal incidence iscollected by the fibers in branch 28. Light impinging the sample atnon-normal angles of incidence is reflected away from the fiber opticsin junction 30 and is therefore not collected by the fibers in branch28. This allows substantial variations in the orientation of the filmsample F as depicted in FIGS. 2 and 3, respectively, and eliminates theneed for critical placement of the sample.

Up to this point in the description attention has been centered upon thebasic operating principle of the invention which involves sending a beamof variable-wavelength monochromatic light in alternate periods througha reference path 18 and a measurement path 20 to the photomultiplier 16,and relatively little attention has been given to the details of theoptical apparatus which causes the light beam to alternate between pathsin this fashion. The reason for presenting the description in thismanner is that it is important to understand in the first instance howthe alternating-path light guiding technique enables the system toallocate all of its calibrating, adjusting and correctional functions tothe digital computer 14, thereby relieving the optical apparatus ofthese functions. This in turn enables the system to utilize an opticalapparatus 10 of simple, low-cost design and construction which mayoperate at much higher speed than conventional spectrophotometersinasmuch as the only essential tasks it now has to perform are togenerate variable-wavelength monochromatic light and cause the same topass alternately through the two light-guiding paths 18 and 20 duringsuccessive periods of wavelength variation. By understanding the aim ofthe invention, which is to subject the entire adjusting, calibrating,and correctional burden of the system to the power of the digitalcomputer 14, one is able to appreciate how this simplification of theoptical equipment can be accomplished while at the same time achievingsuperior performance in comparison with prior systems.

Referring again to FIG. 1, the optical apparatus includes apolychromatic light source 32, which preferably is a type of lamp havinga built-in reflector-lens system, such as would be used ordinarily forilluminating fiber-optic bundles in optical card reading devices. It ispreferred that the lamp 32 be energized by filtered direct current toeliminate short-term variations in its filament temperature and thus inits spectral output. The lamp 32 furnishes light having a span ofwavelengths throughout the visible spectrum. This light passes throughcertain corrective filters 33 and 34 (the purpose of which will beexplained hereinafter) and enters the common end or junction of a splitfiber-optic bundle 36. The functions of the respective legs or branchesof this split bundle 36 will be described presently.

The heart of the optical apparatus 10 is a variable-wavelengthmonochromator in the form of a rotating, generally opaque disk 38 havinga semicircular variable-wavelength interference filter 40 mountedconcentrically therein. The wavelength of light transmitted by thefilter 40 at a given stationary point varies with the angular positionof the filter relative to that point. In an exemplary system of thiskind which presently is in use, the transmission wavelength of thefilter 40 varies through a range of approximately 4000 to 7000 A over anangular span of about 159°. If sampling of the transmitted light beam isassumed to occur every three degrees over that span, this provides 54sampling points, with a change of about 60 A in the transmitted lightbeam's wavelength for each sampling point after the first. In practiceit is not feasible to sample over the full 180° span, due totransitional anomalies which can be expected to occur at the extreme endzones of the filter.

The filter 40 presently being used is a Visible Circular Variable Filterthat is available as a stock item from the Optical Coating Laboratory,Inc., Santa Rosa, California. Since the range of wavelength variationover the usable span is less than a full octave, the measurement of filmthickness may become difficult in a small part of the thicknessmeasurement range (e.g., 1200 to 1400 A in the case of silicon dioxidefilms). This difficulty may be overcome with suitable programming of thecomputer, as described hereinafter, or else by using a more expensivefilter covering a complete octave.

When the filtered light from the source 32, FIG. 1, enters thefiber-optic bundle 36, it contains illuminating components of allwavelengths in the usable spectrum. This light divides through a numberof parallel branches 42 to 46 of the bundle 36. The fibers of theseveral branches are randomized in the common entrance portion of thebundle 36, so that no one branch has a preponderance of its light comingfrom a particular part of the lamp filament whose temperature differsfrom that of other parts of the filament.

The branches 43 and 44 are the paths for conducting the multiwavelengthpolychromatic light to the filter 40. During one-half of its rotationthe filter 40 receives light from path 43, and during the other half ofits rotation it receives light from path 44. While the filter 40 istransmitting light from one of these two paths 43 and 44, the opaqueportion of the disk 38 blocks the light coming from the other one. Thelight beam which is transmitted by the filter 40 in each of thesealternating periods has a wavelength which varies at the rate of about20 A per degree of filter rotation through the range indicated above.

The fiber-optic branches 42 and 45 conduct light to an outer portion ofthe disk 38, which has a series of timing holes 48 extendingcircumferentially in alignment with the ends of the branches 42 and 45.These holes 48 are spaced uniformly along an arc whose angle coincideswith the angle subtended by the usable region of the filter 40. As theseries of holes 48 moves past the exit end of fiber optic 42 or 45, asthe case may be, the intermittent passage of light through these holeswill determine the timing of the sampling points which, as stated above,occur about once every three degrees of rotation. The manner in whichsuch timing or synchronization occurs will be described presently.

Instead of employing the timing holes 48 and the cooperating fiberoptics 42 and 45, an optical incrementing encoder which is commerciallyavailable may be utilized. Such an encoder will increase the number ofsampling points that are available.

The exit end of the fiber optic 46 is aligned circumferentially with astarting hole 50 in the disk 38. As the hole 50 moves into registry withthe exit end of fiber optic 46, a new cycle of wavelength variationcommences, each such cycle including in one half thereof a period duringwhich the filter 40 transmits light from one of the illuminatingbranches 43 and 44, followed by a period in the other half-cycle inwhich it transmits light from the other of these branches.

Positioned behind the rotating disk 38 is a stationary opaque member 52in which there is formed a row of small rectangular slits 54, 55, 56 and57, which are aligned respectively with the ends of the fiber-opticbranches 42, 43, 44 and 45. These fiber-optic end portions are formedinto a configuration conforming with that of the slits. A hole 58 instationary member 52 is positioned for periodic alignment with thestarting hole 50 in disk 38. Behind the slits 55 and 56 are positioned,respectively, the forward end or entrance of the fiber-optic referencepath 18 and the forward end or entrance of the branch 26 of thebifurcated fiber-optic measurement path 26, these forward ends beingshaped to conform with the slits 55 and 56.

It is apparent that as the filter 40 rotates past the exit end of branch43, FIG. 1, a light beam of varying wavelength is passed through thereference path 18 to the photomultiplier detector 16. Similarly, as thefilter 40 rotates past the exit end of branch 44, a light beam ofvarying wavelength is passed through the branch 26 to the sample F, fromwhich it is reflected through branch 28 to the photomultiplier detector16. The slits 55 and 56 and the cooperating portions of the fiber optics43 and 44 are small enough so that at any given instant the light beingtransmitted through either one of these slits from the filter 40 issubstantially monochromatic. It is understood, of course, that inpractice the members 38 and 52 would be positioned much closer togetherthan represented in FIG. 1, which is only a schematic showing.

Although the foregoing description implies that the light from thesource 32 is split into separate beams, one of which is transmittedthrough the slit 55 in the form of varying-wavelength monochromaticlight during one-half of the rotation of the disk 38, and another ofwhich is transmitted through the slit 56 as varying-wavelengthmonochromatic light during the other half of the rotation of disk 38,the result is the same as though a single beam of varying-wavelengthmonochromatic light were being switched alternately to the paths 18 and20 in synchronism with the cyclic repetition of the wavelengthvariation. Conceptually, therefore, the action that takes place will beviewed herein as though it were occurring in the manner last described.

The rotating variable filter 40, being an interference filter, has theunwanted property of passing integer multiples of the desired frequency.To eliminate any problem that otherwise would be caused by thisphenomenon, the filters 33 and 34, FIG. 1, are employed. The function offilter 33 is to prevent the transmission of near-ultraviolet lightthrough the infrared region of the filter 40. Filter 33 has a very sharpcutoff around 3500 A in the ultraviolet portion of the spectrum. Toaugment this effect, a paint or stain may be applied to the red end ofthe rotating filter 40 to attenuate any ultraviolet light received therewithout having any detrimental effects at the blue end. A lemon yellowglass stain has been found satisfactory for this purpose. The filter 34is a smoothing filter to prevent a peaked, noisy response in the greenregion of the spectrum and a falling off in the red and blue regions dueto the characteristics of the photomultiplier 16 and light source 32.Commercially available absorption filters have been used satisfactorilyfor these purposes.

Phototransistor units 60, 62 and 64 placed behind the openings 54, 57and 58, respectively, in member 52 respond to light transmitted throughthese openings for producing appropriate timing or synchronizing signalsin conductors 66, 68 and 70, respectively, leading to an interface unit24. (Unit 24 could be pulsed alternatively by an angular encoder.) Astarting pulse in line 70 marks the beginning of a new pair ofsuccessive wavelength-varying periods, during one of which the referencepath 18 is illuminated, and during the other of which the measurementpath 20 and sample F are illuminated. In each of these periods thetiming pulses in line 66 or 68, as the case may be, cause the responseof photomultiplier 16 to be sampled at regularly occurring timeintervals. The wavelength of the light received by photomultiplier 16will be identical at correspondingly numbered sampling points of therespective periods. Hence, the sampled data passed by the interface 24to computer 14 enables the latter to compare, for each sampledwavelength, the response of the system to light reflected from thesample with its response to light of the same initial intensitytransmitted independently of the sample.

Despite the beneficial effects of the filters 33 and 34 and therandomization of the split fiber-optic bundles, there are inevitabledifferences in the intensities of the light emitted by source 32 andtransmitted by filter 40 at various wavelengths and the correspondingresponses of the photomultiplier 16 at different wavelengths in the sameperiod. In addition to this nonuniform wavelength transmissibility,there may be slow changes in overall performance caused by changes inthe characteristics of the system components due to aging, such asdeclining emissivity of the light source 32 or reduced overallsensitivity of the photomultiplier 16. Only the interference filter 40can be relied upon to retain its properties unchanged. In the presentsystem, instead of requiring the optical apparatus or a human being tomake the necessary calibrations and adjustments to compensate for theseperiodic and aperiodic anomalies, this entire responsibility is placedupon the digital computer 14, which is able to deal with the situationin a far more expeditious fashion than was heretofore possible.

FIG. 4 is a data flow diagram that represents schematically the mannerin which items of data are stored and processed within the computer 14,FIG. 1, in accordance with the invention. In describing theseoperations, reference will be made to the storage and processing of dataas though all of this action were taking place within an assemblageconsisting of registers, counters and arithmetic units constructed asseparate hardware units. While it is convenient to think of theoperation in these terms, it should be kept in mind that FIG. 4 actuallyis intended to depict actions that occur within a programmedgeneral-purpose computer, which uses storage locations with changeableaddresses for these purposes. Subsequently herein there will bepresented a more detailed showing of the programmed method by whichthese functions are accomplished by the computer 14.

Basic to the concept of computer-controlled spectrophotometry disclosedherein is the system's capability of generating, storing and operatingupon two sequences of digital values, one sequence representing innumerical form the sampled responses of the data acquisition apparatus12, FIG. 1, to monochromatic light of various wavelengths when the sameis passed through the reference path 18 directly to the light detector16, and the other sequence representing the sampled responses of theapparatus 12 to monochromatic light of the same respective wavelengthsand intensities when the same has been passed into the measurement path20 and then reflected or transmitted from the sample F to the detector16. In FIG. 4 these two sets of stored numbers are designated EDATA(1,I) and EDATA (2,I), respectively. Thus each of these two data setsrepresents the output of photomultiplier 16 for the series ofwavelengths sampled under control of the timing holes 48, FIG. 1 (or ofan equivalent angular encoder). In some cases each path may be sampledseveral times over several rotations of disk 38 in order to minimize thenoise effects by summing and averaging the inputs for each point. Therespective wavelengths corresponding to the timed sampling points arerepresented by another string of stored numbers designated ALAM in FIG.4. This set of numbers does not change with time and need be measuredonly once for any given variable-wavelength filter 40 and timer, using asuitable set of single-wavelength interference filters for calibrationof the sampling wavelengths. To correct for different attenuations oftransmitted light in the two paths 18 and 20, a fourth string of numbersdesignated CALIB, FIG. 4, is stored in the computer 14. This set ofcorrectional factors is measured only once, using a sample of knowntransmission or reflectance (e.g., bare silicon).

Unique data reduction methods are employed in the disclosed system todetermine from the various sets of stored values described hereinabove,first, the relative reflectance or relative transmission data as afunction of wavelength, then the absolute reflectance or transmissiondata, and finally the calculated thickness or other measured attributeof the sample F. It should be noted that all of the data sets EDATA(1,I), EDATA (2,I), ALAM and CALIB are created and processed as digitalvalue representations, this being the inherent mode of operation of thedisclosed system. Conventional spectrophotometers are inherentlydesigned to operate in analog mode and their outputs generally are notamenable to rapid digital data processing methods. It is possible, butneither fast nor convenient, to obtain digital outputs from conventionalspectrophotometers.

Referring again to FIG. 4, the data sets EDATA (1,I), EDATA (2,I) andCALIB enter into a calculating operation which yields as its output aset of relative reflectance values, designated REFL, for the varioussampled wavelengths. Specifically, the measured EDATA (2,I) values aredivided respectively by the reference EDATA (1,I) values, and theresults are multiplied respectively by the path length calibratingfactors CALIB to obtain the respective REFL values. The division ofEDATA (2,I) by EDATA (1,I), when corrected for path length differences,yields the proportion of incident light reflected from the sample ateach of the sampled wavelengths, i.e., the relative reflectance REFL.

In the conventional spectrophotometric method of measuring filmthickness, relative reflectance is the final output of the apparatus,and the determination of film thickness then must be accomplished bytedious human calculations, which are prone to error since they relyupon the correct human identification of maximum and minimum points inthe curve of relative reflectance versus wavelength and do not involveany other check points in the graphical data. The present system is avery significant improvement over the conventional method, not onlybecause it greatly speeds up the computation of film thickness from theacquired reflectance data but also because it utilizes a large number ofadditional check points between the extrema of the curve in order toimprove the accuracy of the thickness calculation.

According to the present method, the relative reflectance values REFL,FIG. 4, are converted automatically to absolute reflectance values REFby a simple arithmetic process which is based upon the discovery thatall absolute reflectance curves such as 80, 81, 82 and 83, FIG. 6, forfilms of a given material having different thicknesses are tangent to orbounded by common upper and lower limiting curves or wave envelopes 84and 85. Since relative reflectance values are proportional to absolutereflectance values, the relative reflectance values REFL can beconverted to the corresponding absolute reflectance values REF bymultiplying REFL by whatever constant is necessary to bring the relativereflectance curve into tangential relationship with the envelope curves84 and 85 FIG. 6 (or at least one of these curves, as explained laterherein).

The fact that absolute reflectance curves for films of the same materialhaving different thicknesses are bounded by the same wave envelopes maybe deduced from the equation for absolute reflectance given below:

Reflectance = [R₁ ² + R₂ ² + 2R₁ R₂ cos (2β - φ)]/[1 + R₁ ² R₂ ² + 2R₁R₂ cos (2β - φ)]

where

β = (2π dN₁)/λ

d = film thickness

φ = tan⁻ ¹ (2N₁ K₂)/(N₁ ² -N₂ ² -K₂ ²)

R₁ ² = [(n₁ - n₀)/(n₁ + n₀)]²

r₂ ² = [(n₂ - n₁)² + k₂ ² ]/[(n₂ + n₁)² + k₂ ² ]

n₁ = index of refraction for film

N₂ = index of refraction for substrate

N₀ = ambient index of refraction

K₂ = extinction coefficient

Analysis of this equation shows that all curves of absolute reflectanceversus wavelength must be bounded by envelope curves which are producedby setting the cosine term equal to +1 and -1, respectively.

When the sampled data points REF for the curve of absolute reflectanceversus wavelength are obtained, the following equation then is solvedexplicitly for the film thickness at each data point:

Thickness = (λ/4πN₁) (φ + 2π (ORD) + cos⁻ ¹ [REF (1+R₁ ² R₂ ²)-R₁ ² -R₂² /2R₁ R₂ (REF - 1)])

where ORD = fringe order, i.e., a count of the number of maximum-minimumvariations (or to be more accurate, tangency points) through which thecurve theoretically would have gone from its beginning at infinitewavelength to the point in question.

The fringe order number ORD is unknown but can be determined relativelyeasily by a systematic trial-and-error procedure, making use of the factthat if all measurements were perfect, the thickness determined at eachdata point would have the same value, if the proper order number werechosen for each point. Thus, an initial estimate is made of the ordernumber for the point at one end of the available curve; then thethickness is calculated at all points to see how much deviation or"scatter" there is among the various values obtained. After this trialhas been made, the initial order number is incremented by 1, and thethickness is recalculated for all points to compare the scatter withthat previously obtained. This procedure is repeated until a set ofresults with minimum scatter is obtained, at which time it is concludedthat the thickness thus calculated is the true film thickness. All ofthese calculations, complex as they seem, are performed at extremelyhigh speed and with ease by the computer 14 and therefore take verylittle time.

The above procedure works very well for films that are not less than 700A thick. Films thinner than this are analyzed by the use of a curvefitting technique, identified hereinafter as THINI, which is one of thesubroutines that are available in the programming of the computer 14.Another special situation may arise where the film thickness is betweencertain values (such as 1250 to 1400 A for silicon dioxide) due to thefact that the reflectance curves for films in this thickness range donot manifest any extrema for incident light in the wavelength span from4000 to 7000 A. This problem can be handled either by increasing thewavelength span (using a more expensive rotatable filter 40) or bymodification of the programming as indicated hereinafter.

A program under which a system of this kind has been successfullyoperated consists of FORTRAN and FORTRAN-callable subroutines, theprincipal ones of which are shown in FIGS. 7A to 10B, inclusive. Aflowchart depicting that portion of the program which includes theseprincipal subroutines is presented in FIGS. 5A - 5E. The remainingsubroutines of the program are not specifically disclosed herein, sincetheir construction would be apparent to a programmer of average skill.

The subroutines discussed herein are as follows:

DATAI -- FIGS. 7A and 7B -- This is a FORTRAN-callable subroutinewritten in assembly language of a particular computer. The example ofDATAI shown herein was written for an 1130 data processing systemhookup. Its function is to acquire data from the two paths 18 and 20,FIG. 1, as described hereinabove and store the resulting data sets EDATA(1,I) and EDATA (2,I), FIG. 4.

THIKI -- FIGS. 8A to 8F -- This is a FORTRAN program which performs themajor part of the data processing operations involved in determining thefilm thickness, calling the subroutines DATAI and ORDR as required. Thevarious functions of THIKI are indicated on FIGS. 5A, 5C, 5D and 5E ofthe flowchart, or stated more generally, they comprise the operationsindicated in FIG. 4 commencing with the calculation of the relativereflectance values REFL. THIKI also handles the special situations whicharise whenever the thickness of the measured film sample lies in a rangesuch that its reflectance curve has no maximum or minimum point withinthe usable wavelength span of the incident light.

ORDR -- FIGS. 9A to 9D -- This subroutine performs various graphicanalysis functions, indicated on FIG. 5B and part of FIG. 5C, which willbe described in more detail hereinafter. It also is able to detect whenthe film is too thin or be calculated by the THIKI subroutine, and itcalls the THINI subroutine under these circumstances.

THINI -- FIGS. 10A and 10B -- This subroutine performs a second-orderleast-squares fit to the reflectance curve and does a table lookup ofthe film thickness.

Referring now to FIG. 5A, wherein the boxes containing the flowchartsteps are individually numbered for convenience, the steps 600 and 601relate to the data acquisition subroutine DATAI. After an initial waitfor the start command (step 600), the system executes the DATAIsubroutine for acquiring the data sets EDATA (1,I) and EDATA (2,I), FIG.4, from the reference path 18 and sample measurement path 20,respectively. A typical program for carrying out these DATAI functions,written in the assembly language of the IBM 1130 data processing system,is shown in FIGS. 7A and 7B.

The program now exits from the DATAI subroutine and enters the THIKIsubroutine, an exemplary FORTRAN programming of which is shown in FIGS.8A-8F. Referring back to FIG. 5A, the first step in determining filmthickness (602) is to divide the sample path data EDATA (2,I) by thereference path data EDATA (1,I). In most instances this will be done aspart of the procedure for measuring the relative reflectance of anunknown sample which is undergoing measurement. During the initialcalibration of the system, however, a sample whose reflectancecharacteristic is already known (bare silicon, for example) is measuredin order to compare its known reflectance values at the various samplingwavelengths with the respective quotients obtained in step 602, whichtheoretically should equal these known values.

At step 603, FIG. 5A, the system inquires whether it is presentlyoperating in a calibration mode or a measurement mode. When operating inthe calibration mode, the values obtained in step 602 are divided by therespective values which they should have had (step 604) and the resultsare stored as calibration factors (step 605). The operation then loopsback to the starting step 600. If the system is operating in itsmeasurement mode, the operation branches from the decision point 603 tostep 606, where the quotients obtained in step 602 are respectivelydivided by the calibration factors obtained in the previously executedstep 605 in order to obtain the corrected values of relativereflectance, denoted as REFL in FIG. 4.

At step 607, FIG. 5A, the system makes a rough estimate of the number of"fringes" (i.e., maxima) that are present in the reflectance curve, inthe sense of detecting whether this number is low or not low. If it islow (as may be the case in small parts of the measurable thicknessrange), then the number of data sampling points is reduced in order tominimize the possibility of noise signals being mistaken for extrema. Ifthe number of fringes is not low, then there is much less likelihood ofspurious noise pulses being mistaken for extrema, and the normal numberof sampling points may be used. The program makes this determination atstep 608.

Then at step 609, the system determines whether smoothing of the dataobtained in step 606 is desired, the decision being made in accordancewith the rough estimate obtained in step 607. The part of the THIKIprogram which deals with the steps numbered 607-609, FIG. 5A, is shownat the bottom of FIG. 8B and in FIG. 8C. If data smoothing is desired,any of several standard techniques is employed for this purpose asindicated at step 610. If smoothing is not desired, step 610 is omitted.

At this point the system exits for a time from the THIKI subroutine andenters the ORDR subroutine, FIGS. 9A-9D, the functions of which areindicated generally in boxes 611-618 of the flowchart, FIGS. 5B and 5C.First the operation proceeds to step 611, FIG. 5B, at which the programmakes one of several checks for bad points in the data. Such a point maybe recognized by the erratic behavior of the slope or derivative of thecurve in its vicinity. In the next step 612 a check is made for datapoints taken at wavelengths where the particular material in the sampleis known to be highly absorbing, for example, the blue end of thespectrum in the case of photoresist coatings.

The system now is ready to find the multiplying factor or constant forconverting the relative reflectance (REFL) curve to the absolutereflectance (REF) curve. This is done at step 613, if it can beaccomplished at all. If the material sample happens to be in a thicknessrange where no extrema exist in the wavelength span being used, an exactdetermination of the constant is not possible. At step 614 it isascertained whether any points of tangency have been found. A point oftangency will be found close to each one of the extrema, if there areany. If such points are found, they are notated at step 615 of theprocedure.

If no tangency points were found, the system then must determine whetherthis was due to the fact that the film was thinner than say, 700 or 750A, or because its thickness fell within some other range where noextrema exist. This determination is made at step 616. If the slope ofthe reflectance curve is positive, the film thickness is in the verythin range, and a special subroutine THINI (the programming of which isshown in FIGS. 10A and 10B) is called to apply a curve fitting techniqueto the data. If the reflectance curve slope is negative, the filmthickness is in the other critical range, and the operation proceeds tostep 617 of the THIKI subroutine, where a special calculation isperformed to determine a multiplier at the end of the reflectance curvewhich is closest to being a point of tangency on one of thepre-established wave envelopes. As mentioned above, this step 617 may beomitted if the rotating interference filter 40, FIG. 1, has asufficiently wide wavelength span to insure the inclusion of at leastone extremum in the reflectance curve.

The mathematical procedure for determining the proper multiplierconstant is not difficult. The wave envelope values for the respectivesampling wavelengths will have been determined and stored beforehand inthe computer 14. The sampled, good values of the reflectance curve, ascorrected, are then divided by the respective reflectance values foreach of the wave envelopes to find a quotient which most nearlyapproaches 1. This quotient, inverted, subsequently becomes themultiplier constant for the whole curve.

After step 615 or 617 has been performed, as the case may be, theoperation advances to step 618, FIG. 5C, where a notation is made of theincremental slope at each sampled data point. This information will beneeded subsequently in the thickness calculation (the mathematicalformulas for which were presented hereinabove) for determining theproper sign to be used in the arc cosine evaluation. Step 618 is thelast phase of the ORDR subroutine.

Having completed this step, the procedure now reenters the THIKIsubroutine, and at step 620, FIG. 5C, a branching decision is madeaccording to whether the film's absorption of light at any of thewavelengths can be ignored. This, of course, depends upon the materialin the sample being measured and is information that must be supplied tothe system beforehand. Some dielectric materials such as SiO₂ havenegligible absorption. Others such as photoresist may absorb heavily inone particular part of the wavelength spectrum. If a nonabsorbingmaterial is being measured, the relative reflectance curve values (REFL)are multiplied by the multiplier found in step 613 or 617, FIG. 5B, thisaction occurring at step 621, FIG. 5C. If an absorbing material is beingmeasured, the multiplier is determined on the basis of fitting thereflectance curve to the upper wave envelope only (step 622), because itmust be made tangent to at least one envelope, and the upper one is themore important.

The absolute reflectance curve values having been found, there nowcommences the cut-and-try process described hereinabove for finding thetrue order numbers of the fringes in this reflectance curve, which willbe needed in the thickness calculation. Depending upon the number oftangency points previously determined (step 615, FIG. 5B), an initialestimate of the fringe order at one end of the curve is made by thesystem (step 623). Then, at step 624, a trial calculation of filmthickness is made at each wavelength sampling point where there is agood reflectance value, using well-known formulas as given hereinaboveand changing the order number at each point of tangency to the upperenvelope.

At step 625, FIG. 5C, the average of the several thickness valuescalculated in step 624, along with their standard deviation (or"scatter"), are calculated. If this is the first pass through thethickness calculation (decision point 626), then the starting orderguess is incremented (step 627, FIG. 5D), and the process loops back tostep 624, FIG. 5C. The reason for this is that there must be at leasttwo sets of thickness calculations for comparison purposes. Steps 624and 625 now are repeated to derive a new set of thickness values,together with their average value and the standard deviation.

At step 628, FIG. 5D, the current standard deviation is compared withthe one previously obtained. If there has been an improvement inaccuracy, as reflected by a reduction of the deviation value, anothertry is made for still greater accuracy by again incrementing thestarting order guess (step 626) and repeating the calculation in steps624 and 625. Ultimately a condition is reached where the deviation valuestarts to increase after having progressively declined. This indicatesthat the immediately preceding thickness calculation was the correctone, since it involved the least amount of scatter. The program nowexits from the thickness calculation loop 624-628 to step 629, where theaverage thickness calculated in the pass just preceding the current oneis preserved as the ostensibly correct result.

The operation by now has progressed to a point where the answer may beprinted out, if no refinement of the calculations is desired. This is adecision made by the operator, who has set a console switch to a chosenposition indicative of either refinement or no refinement. At step 630the setting of this console switch is tested and if no refinement iscalled for, the thickness value saved at step 629 is printed out as thefinal answer (step 631). The system then is initialized in preparationfor another thickness measuring operation upon a new sample (step 600,FIG. 5A).

If refinement of the thickness calculations is called for at step 630,FIG. 5D, then an iteration involving steps 632 to 640, FIGS. 5D and 5E,is initiated. The purpose of this iteration is to make certainexperimental, small, vertical displacements of the REF curve to seewhether such action further reduces the amount of deviation or scatterin the thickness calculations throughout the wavelength sampling span,thereby indicating a truer fit of the absolute reflectance curve to thedata. At step 632 the starting order number last calculated (step 627 inthe last pass) is reset to the next lower number, which is the correctone for the sample under consideration. Now a new set of absolutereflectance values (REF) is obtained by a process which involves, first,decreasing the multiplier constant previously used (step 621, FIG. 5C)by some initial trial amount such as 20% (step 633, FIG. 5D). Therelative reflectance values REFL found in step 606, FIG. 5A (as modifiedby the subsequent correctional steps) now are multiplied by this newconstant to calculate a new curve or table of reflectance values for thesample (at step 634).

The steps 635, 636 and 637 which now follow are essentially the same asthe steps 624, 625 and 626 which were previously executed in order tominimize the scatter. A tentative set of thickness values, together withan average thickness value based thereon and a standard deviation value,are calculated. On the next pass through this smoothing iteration, theprevious multiplier is incremented by a small amount such as 4% (steps637 and 638), and a new set of thickness values is calculated (steps634-636). The current and previous deviations are compared (step 639),and if there has been an improvement, the multiplier again isincremented by 4% (step 638), and another try is made to reduce thedeviation still further. When no further improvement is possible (steps639 and 640), the final thickness value is printed (step 641).

It will be appreciated that all of the operations depicted in FIGS. 5A -5E are performed in a length of time which is insignificant incomparison with the time consumed by a conventional spectrophotometricmethod of measuring film thicknesses, and the results are far moreaccurate since more data points are being considered and are beingchecked more thoroughly than before. Inasmuch as these operations areconducted by a highly flexible and versatile digital computer, theoptical apparatus is freed of all tasks except the simple one ofacquiring the raw optical data, uncorrected for any variations of lightintensity with either the periodic changes of wavelength or theaperiodic aging of the system components, such factors now being takencare of by the programmed internal processes of the computer, along withadditional refining techniques that are feasible because thespectrophotometer now has the power of the computer available to it.

Beside its use as a film thickness measuring tool, the disclosed systemwill lend itself to other applications of spectral analysis, such asexamining the reflectance characteristics of printing inks or otherlight-reflecting substances or the absorption characteristics of lightfiltering substances. The disclosed system moreover is capable ofmodification to suit special thickness measuring conditions. Forexample, if all of the measurements to be made are known to fall withincertain limits, it may be feasible to eliminate certain routines such asTHINI or the special calculation denoted as step 617 in FIG. 5B. If thenumber of data points to be used by the system is fixed beforehandinstead of being determined by the system itself as at step 608, FIG.5A, this will eliminate the need for the software and hardware which areinvolved in this determination. Various other modifications andimprovements apparent to those skilled in the art can be effectedwithout departing from the scope and spirit of the invention.

What is claimed is:
 1. A method of operating a spectrophotometric systemunder the control of a digital computer to determine an attribute of agiven material sample which is subject to spectral analysis, said methodcomprising the steps of:a. conducting to said sample, during each of aseries of nonadjacent time periods, a beam of variable wavelengthmonochromatic light furnished by a given source, the wavelength of saidbeam varying at a given rate through a specified range of wavelengthvalues during each of said periods; b. conducting to a light detector insaid system the light which comes from said sample when it is impingedby said beam; c. conducting directly to said light detector, during timeperiods intervening the periods specified in step a, the beam ofmonochromatic light furnished by said source, the wavelength of whichvaries at said given rate through said specified range during each ofsaid intervening periods; d. converting the output of said detectorduring each of the periods specified in steps a and c to a sequence ofstored digital values representing the respective intensities of lightdetected at a series of regularly timed intervals throughout therespective one of said periods; e. operating said computer to derivefrom the sequences of values stored during any pair of successive stepsa and c a new sequence of numbers representing the calculated opticalresponses of said sample to incident light having the wavelength of saidvariable-wavelength beam at each of said timed intervals under ahypothetical condition where it is assumed that the incident light hasuniform intensity for all wavelengths in said range and all systemcomponents have constant operating characteristics; and f. operatingsaid computer to determine from said derived sequence of numbers theattribute of said sample which is being measured.
 2. A method ofoperating a spectrophotometric system under the control of a digitalcomputer to determine the thickness of a film of given dielectricmaterial, said method comprising the steps of:a. conducting to saidfilm, during each of a series of nonadjacent time periods, a beam ofvariable-wavelength monochromatic light furnished by a given source, thewavelength of said beam varying at a given rate through a specifiedrange of wavelength values during each of said periods; b. conducting toa light detector in said system the light reflected from said film as itis illuminated by said beam; c. conducting directly to said lightdetector, during time periods intervening the periods specified in stepa, the beam of monochromatic light furnished by said source, thewavelength of which varies at said given rate through said specifiedrange of values during each of said intervening periods; d. operatingsaid computer to convert the output of said detector during each of theperiods specified in steps a and c to a sequence of stored digitalvalues representing the respective intensities of light detected at aseries of regularly timed intervals throughout the respective one ofsaid periods; e. operating said computer to derive from the sequences ofdigital values stored during any pair of successive steps a and c, asdescribed above, a series of numbers each representing the relativereflectance of said film when illuminated by monochromatic light havingthe wavelength of said beam at a respective one of said timed intervalsunder a hypothetical condition where it is assumed that the incidentlight has uniform intensity for all wavelengths in said range and allsystem components have constant operating characteristics; and f.operating said computer to determine from said relative reflectancevalues the thickness of the film being measured.
 3. In aspectrophotometric system having data acquisition means for generatingdigital value-representing signals in response to sensed optical inputsand also having a digital computer provided with data storage means anddata processing means which are responsive to the output of said dataacquisition means, a method of operating said system to measure thethickness of a film sample of given dielectric material, comprising thesteps of:a. conducting to said film sample, during each of a series ofnonadjacent time periods, a beam of variable-wavelength monochromaticlight furnished by a given source, the wavelength of said beam varyingat a given rate through a specified range of wavelength values duringeach of said periods; b. conducting to a light-sensitive input device insaid data acquisition means the light which is reflected from saidsample when it is impinged by said beam; c. conducting directly to saidlight-sensitive device, without impinging said sample and during timeperiods intervening those specified in step a, the beam ofvariable-wavelength monochromatic light furnished by said source, thewavelength of which varies at said given rate through said specifiedrange of values during each of said intervening periods; d. operatingsaid computer to receive and store in said data storage means, atregularly timed intervals in each of the time periods described in a andc, the digital values generated by said data acquisition means, therebyto store at least two sets of numbers, one set representing therespective intensities of light detected by said light-sensitive deviceat said timed intervals during a period when the variable-wavelengthlight beam is being conducted directly to said device, and the other setof numbers representing the respective intensities of light detected bysaid device at said timed intervals during a period when thevariable-wavelength light beam is impinging said film sample; e. storingin said data storage means a set of numbers representing the respectivewavelengths of the light beam at the respective ones of said timedintervals throughout any of said periods; f. operating said dataprocessing means to derive from all of the stored number sets recitedhereinabove a table of values representing the manner in which therelative reflectance of said film sample varies with respect to thewavelength of the incident light beam under a hypothetical conditionwhere it is assumed that the incident light has uniform intensity forall wavelengths in said range and all system components have constantoperating characteristics; g. operating said data processing means tomultiply the relative reflectance values in said table by a factor whichwill convert the curve of relative reflectance versus wavelength to acurve which is at least approximately tangent to predefined upper andlower wave envelopes that bound the curves of absolute reflectanceversus wavelength for all films of the given material having thicknesseswithin a predetermined thickness range; and h. operating said dataprocessing means to compute from the multiplied reflectance values thethickness of said film sample.
 4. A computer-controlledspectrophotometric system for measuring a property of a given materialsample which is subject to spectral analysis, said system comprising:a.cyclically operable illuminating means for furnishing a beam ofmonochromatic light, the wavelength of which varies periodically at agiven rate through a specified range of wavelength values; b. a lightdetector; c. first light guiding means for conducting said light beamthrough a first path directly to said light detector during nonadjacentones of the periods in which the wavelength of said light beam undergoesvariation through said range of values; d. second light guiding meanseffective during periods intervening said nonadjacent periods forconducting said light beam through a second path having an initialportion that directs said beam to said sample and a final portion thatconducts light from said sample to said detector; e. signal convertingand data transfer means responsive to the output of said detector andoperating in a timed relationship with the cyclic operation of saidilluminating means for producing sequences of digital valuerepresentations, each such sequence denoting the variation of detectedlight intensity with respect to the wavelengths of said beam at a seriesof regularly timed intervals within each of said periods; and f. adigital computer having data storage means for storing the sequences ofdigital value representations produced by said means e during at leasttwo successive periods when said detector is receiving light throughsaid first path and said second path, respectively, and having dataprocessing means to calculate from such stored data the opticalresponses which would have been obtained from said sample at said timedintervals if the conditions had been such that the intensity of thevariable-wavelength incident light were uniform at all wavelengths insaid range and the operating characteristics of the components of saidsystem were constant, thereby providing a new set of corrected valuesfrom which the measurement of said sample property can be accuratelydetermined.
 5. A spectrophotometric system as set forth in claim 4wherein said illuminating means a comprises the following elements:a1. asource of polychromatic light; a2. first light conducting means fordirecting polychromatic light from said source to an exit pointoptically aligned with but spaced from the entrance to said first pathin said first light guiding means; a3. second light conducting means fordirecting polychromatic light from said source to an exit pointoptically aligned with but spaced from the entrance to said second pathin said second light guiding means; and a4. a rotatingvariable-wavelength interference filter positioned so that it rotatesalternately past the entrance to said first path and the entrance tosaid second path, the wavelength of the monochromatic light transmittedby said filter to either path depending upon the angular position ofsaid filter.
 6. A spectrophotometric system as set forth in claim 5wherein each of said light conducting means and light guiding means is afiber-optic bundle.
 7. A system as set forth in claim 6 wherein saidsecond light guiding means (d) is bifurcated fiber-optic bundle, onebranch of which constitutes said initial portion, and the other branchof which constitutes said final portion.
 8. A spectrophotometric systemas set forth in claim 4 which includes timing means under the control ofsaid illuminating means for causing said signal converting and datatransfer means to sample the output of said light detector atpredetermined wavelength increments during each of the periodicvariations of the monochromatic beam wavelength.
 9. A spectrophotometricsystem as set forth in claim 4 wherein said computer is programmed todivide each of the values in the sequence that was received through saidsecond path by the corresondingly positioned value in the sequence thatwas received through said first path.
 10. A spectrophotometric system asset forth in claim 4 wherein said second light guiding means is arrangedso that light passed through said initial portion thereof is reflectedfrom said sample and passed through said final portion thereof to saiddetector.
 11. A system as set forth in claim 10 wherein said computer isprogrammed to divide each of the values in the sequence that wasreceived through said second path by the corresondingly positioned valuein the sequence that was received through said first path, thereby toyield said sequence of corrected values representing the relativereflectance of said sample at the respective wavelengths assumed by saidbeam at said timed intervals.
 12. A system as set forth in claim 11wherein said computer is programmed to multiply each of the values insaid relative reflectance sequence by a factor which will cause themultiplied values to define a curve that is tangent to at least one of apair of predetermined wave envelope curves which bound all curves ofabsolute reflectance versus wavelength for the given material, therebyproviding a set of absolute reflectance values for said respectivewavelengths.
 13. A system as set forth in claim 12 wherein said computeris programmed to calculate from said sequence of absolute reflectancevalues the thickness of said sample.
 14. In a computer-controlledspectrophotometric system of the kind wherein a light detector isarranged to respond to light of periodically varying wavelength which isreceived alternately through a sample measurement path and a directreference path, the combination comprising:a. a rotatablevariable-wavelength filter having a light-transmitting portion with anangular span not exceeding 180° positioned so that it transmits lightinto the respective entrances of said measurement path and saidreference path during different periods in its rotation, the wavelengthof the light transmitted into either of said paths at any instantdepending upon the angular position of said filter relative to the pathentrance; b. a polychromatic light source; and c. means providing lightconducting paths leading from said light source to exits on the side ofsaid filter which is farthest from said measurement path and referencepath entrances, said path exits being respectively aligned opticallywith said entrances, whereby the rotation of said filter causespolychromatic light emerging from said path exits to be transmitted bysaid filter as monochromatic light of varying wavelength to saidmeasurement path and said reference path, respectively, during differentperiods in the rotation of said filter.
 15. The combination set forth inclaim 14 wherein said means c is a split fiber-optic bundle havingbranches leading respectively to said exits and having a common portionadjacent to said light source wherein the fibers of said branches have arandomized distribution. .Iadd.
 16. A method of operating aspectrophotometric system under the control of a digital computer todetermine an attribute of a given material sample which is subject tospectral analysis, said method comprising the steps of:a. conducting tosaid sample, during each of a series of nonadjacent time periods, a beamof light which includes at least part of the light furnished by a givenpolychromatic light source; b. conducting to a light detector in saidsystem a beam of light which comes from said sample when it isilluminated by the first-mentioned light beam during each of saidnonadjacent periods; c. conducting to said detector through a path thatis independent of said sample, during time periods intervening thenonadjacent time periods in the first-mentioned series, a beam of lightwhich includes at least part of the light furnished by said lightsource; d. selectively filtering, in a time-varying fashion, the lightwhich passes from said source to said detector by way of said sampleduring each of said first series of time periods and the light whichpasses from said source to said detector independently of said sampleduring each of said intervening periods for causing the light which isthereby furnished to said detector to be a monochromatic light beamwhose wavelength varies as a given function of time through a specifiedrange of wavelength values during each of said nonadjacent periods andeach of said intervening periods; e. converting the output of saiddetector during each of said nonadjacent and intervening periods to asequence of stored digital values representing the respectiveintensities of light detected at a series of predefined time intervalsthroughout the respective one of said periods; f. operating saidcomputer to derive from the sequences of values stored during any pairof successive periods, including one of said first series of nonadjacentperiods and one of said intervening periods, a new sequence of numbersrespectively representing the calculated optical responses of saidsample to light having the wavelength of said monochromatic light beamat said predefined time intervals under a hypothetical condition whereit is assumed that the light conducted to said sample has uniformintensity for all wavelengths in said range and that all systemcomponents have constant operating characteristics; and g. operatingsaid computer to determine from said derived sequence of numbers theattribute of said sample which is being measured. .Iaddend..Iadd.
 17. Amethod as set forth in claim 16 wherein the sample is a dielectric film,and the attribute being measured is the film thickness. .Iaddend..Iadd.18. A method as set forth in claim 16 wherein the light conducted tosaid detector from said sample is light which has been reflected by saidsample from said first light beam. .Iaddend. .Iadd.
 19. A method as setforth in claim 18 wherein the sample is a dielectric film, and theattribute being measured is the film thickness. .Iaddend..Iadd.
 20. Acomputer-controlled spectrophotometric system for measuring a propertyof a given material sample which is subject to spectral analysis, saidsystem comprising:a. a source of polychromatic light; b. a lightdetector; c. first light guiding means for conducting light from saidsource through a first path to said light detector, said first pathincluding an antecedent portion for directing light to said sample and asubsequent portion for directing light from said sample to saiddetector; d. second light guiding means for conducting light from saidsource through a second path to said light detector, said second pathbeing independent of said sample; e. optical filtering means positionedin each of said light paths for converting polychromatic light tomonochromatic light having a wavelength that varies as a given functionof time through a predetermined range of wavelength values during eachof a series of time periods, said filtering means being so arranged thatmonochromatic light of any particular wavelength in said range isdirected to said detector through one of said paths at a time that isdisplaced by a predetermined amount from the time at which monochromaticlight of said particular wavelength is directed to said detector throughthe other of said paths; f. signal converting and data transfer meansresponsive to the output of said detector and operable in timedrelationship with said filtering means for producing sequences ofdigital value representations, each such sequence denoting the variationof detected light intensity with respect to wavelength at a series ofpredefined time intervals within each of said periods; and g. a digitalcomputer for storing the sequences of digital value representationsproduced by said signal converting and data storage means as it receivesvariable-wavelength monochromatic light through said first path and saidsecond path, respectively, and for calculating from such stored data theoptical responses which would have been obtained from said sample if theconditions had been such that the intensity of the light conducted tosaid sample were uniform at all wavelengths in said range and theoperating characteristics of the system components were constant,thereby providing a new set of corrected values from which themeasurement of said sample property can be accurately determined..Iaddend..Iadd.
 21. A system as set forth in claim 20 wherein saidoptical filtering means is a rotatable variable-wavelength filter sopositioned that during its rotation it moves alternately across saidfirst path and said second path, the wavelength of the monochromaticlight which is transmitted by said filter to the succeeding portion ofeach path at any instant depending upon the angular position of saidfilter. .Iaddend. .Iadd.
 22. A system as set forth in claim 20 whereineach light guiding means includes a fiber-optic bundle, said first lightguiding means having a bifurcated fiber-optic bundle, one branch ofwhich is in the antecedent portion of said first path, and the otherbranch of which is in the subsequent portion of said first path, wherebysaid detector receives light which is reflected from said sample..Iaddend..Iadd.
 23. A system as set forth in claim 20 wherein said firstlight guiding means is arranged so that light passed through saidantecedent portion thereof is reflected from said sample and is thenpassed through said subsequent portion thereof to said detector..Iaddend..Iadd.
 24. A system as set forth in claim 23 wherein saidcomputer is programmed to divide each of the values in the sequence thatwas received through said first path by the correspondingly positionedvalue in the sequence that was received through said second path,whereby said sequence of corrected values will represent the relativereflectance of said sample at the respective wavelengths assumed by saidbeam at said timed intervals. .Iaddend..Iadd.
 25. A system as set forthin claim 24 wherein said computer is programmed to multiply each of thevalues in said relative reflectance sequence by a factor which willcause the multiplied values to define a curve that is tangent to atleast one of a pair of predetermined wave envelope curves which boundall curves of absolute reflectance versus wavelength for the givenmaterial, thereby providing a set of absolute reflectance values forsaid respective wavelengths. .Iaddend. .Iadd.
 26. A system as set forthin claim 25 wherein said computer is programmed to calculate from saidsequence of absolute reflectance values the thickness of said sample..Iaddend.